Response Of Maize Yield Formation To The Variation In Sowing Date Period Period And The Regulating Effect Of Nitrogen

Under the background of global climate change,the average temperature in Northern China is continuously increasing,and the frequency and intensity of extreme high temperature weather continue to increase.Especially on North China Plain,t he high temperature weather generally occurs in July and August,and it can reach 35℃and above,and some areas can even reach more than 40℃.This period is the critical monment of before and after the flowering of summer maize,and high temperature stress often cause s maize yield reduction of more than 15%or even no harvest.Previous studies have shown that adjusting sowing date and optimizing nitrogen fertilizer management can al leviate the effects of extreme high temperature before and after flowering on the growth and development of maize.However,the regulation effects of maize sowing date and nitrogen fertilizer interaction on temperature conditions,yield formation and nitro gen use efficiency during maize growth period,and the physiological mechanism of alleviating maize high temperature stress before and after flowering are not clear.Usin g Zheng Dan ZD958（heat tolerant type）and Xianyu XY335（heat sensitive type）as mater ials,a field experiment was carried out in Xinxiang Experimental Base of Institute of Crop Science,Chinese Academy of Agricultural Sciences from 2019 to 2021 to study the effects of the interaction of sowing date and nitrogen fertilizer on leaf photosynt hetic characteristics,antioxidant characteristics,plant matter accumulation,grain filling characteristics,activities of enzymes related to carbon and nitrogen metabolism and endogenous hormone content under post-anthesis high temperature stress.Combined with the indoor precise temperature control experiment,the physiological mechanism of nitrogen regulating the yield formation of maize under post-anthesis high temperature stress was analyzed,which provided a theoretical basis for stress-resistant cultivation of maize in North China Plain.The main results are as follows:1.Adjusting the sowing date can alleviate the effect of before and after flowering high temperature stress on maize yield formation by changing the temperature conditions of maize growth period.Compared with the normal sowing date（SD2）,20 days in advance of sowing date（SD1）advanced the flowering date by 17 days,while 20 days delay in sowing date（SD3）delayed the flowering date by 18 days.Therefore,both SD1 and SD3 significant ly decreased the daily average temperature(T_mean),daily high temperature(T_max)and daily low temperature(T_min)during the V12-R3 period,with an average decrease of 2.0,2.2,2.7℃in SD1 and 3.6,3.5,4.7℃in SD3,respectively,and then affecting the photosynthetic characteristics and dry matter accumulation of maize.The photosynthetic indexes such as Pn,Tr,Gs and LAI in leaves of SD1 treatment V12 and R1 period and SD3 treatment R1 and R3period were significantly higher than those of SD2,which s ignificantly increased the dry matter accumulation of SD1 and SD3 treatments.As a result,the average annual yield of ZD958 and XY335 of SD1 and SD3 treatments was 23.9%and 14.7%,23.8%and 13.9%higher than that of SD2,respectively.Regression analysis showed that Tmax and Tmin from V12 to R3 period were lower than 35.8 and 23.9℃,35.6 and 23.8°C,respectively,which was a necessary condition for summer maize to obtain higher dry matter accumulation.2.The interaction between sowing date and nitrogen fertilizer can further alleviate the effect of high temperature stress and improve maize yield and nitrogen use efficiency.The results showed that with the increase of nitrogen application rate,the yield of N3(240 kg N hm^-2)treatment under SD1 and SD3 sowing date was the highest,and there was no significant difference between N3(240 kg N hm^-2)and N4 treatment(360 kg N hm^-2).Under SD2 sowing date,the yield of N4 treatment was the highest,and the highest yield of SD1and SD3 with N4 treatment was significantly higher than that of SD2.The difference s in yields were mainly due to the increase of leaf area index,dry matter accumulation rate and root length caused by higher nitrogen accumulation under SD1 and SD3 sowing date.Under the sowing date of SD1 and SD3,the partial productivity,agronomic use efficiency and apparent use efficiency of nitrogen fertilizer of each nitrogen fertilizer treatment were significantly higher than those of SD2.In addition,under SD2 sowing date,the nitrogen and dry matter accumulation and yield of N4 treatment were significantly higher than those of other nitrogen fertilizer treatments,indicating that sufficient nitrogen could increase maize dry matter accumulation and nitrogen accumulation under high temperature st ress,thus facilitating higher yield.3.Nitrogen alleviates the effect of high temperature stress on grain filling by affecting leaf photosynthetic characteristics and grain carbon and nitrogen metabolism.High temperature stress at filling stage decreased the activities of phosphoenolpyruvat e carboxylase and ribulose bisphosphate carboxylase in leaves,which led to the decrease of photosynthetic rate,stomatal conductance and transpiration rate of leaves.And the contents of soluble protein,chlorophyll and carotenoid and the activities of an tioxidant enzymes in leaves decreased,while the content of malonaldehyde increased.However,with the increase of nitrogen application rate,the photosynthetic characteristics and antioxidant enzyme activities of leaves showed an upward trend,thus faciliatating a higher production capacity of photosynthetic matter.In addition,with the increase of nitrogen application rate,the activities of soluble sugar,protein and carbon metabolism related enzymes in grains showed an upward trend,the contents of aux in and gibberellin in endogenous hormones increased,and abscisic acid decreased,thus facilitating a higher ability of matter accumulation in grains.4.In order to further reduce the amount of nitrogen fertilizer applied,the regulation effects of drip irrigation on maize yield and nitrogen use efficiency were studied given that nitrogen could alleviate the high temperature stress before and after flowering.Drip irrigation and fertilization kept high water content and mineral nitrogen content in 0-40cm shallow soil.Under the condition of 20%（N3）reduction of nitrogen application,the yield was 7.3%higher than the maximum yield under flood irrigation,and nitrogen partial productivity and nitrogen agronomic use efficiency increased by 30.4%and 28.6%,respectively.Therefore,the establishment of drip irrigation,fertilizer-saving and stress-resistant technical measures of drip irrigation can provide technical support for stress-resistant and efficient cultivation in North China Plain.